Bringing a Shank to a Knife Robbery

I've seen plenty of news reports of people robbing retail locations with guns, and there have even been some with knives, but I was surprised to see a news report of an attempted shank robbery. The word shank is a slang term for a shiv, which is a makeshift knife. (I am not sure why the news used the term "shank.") I've seen enough films with prison settings to know that shivs are commonly used in prison, since knives and guns are not readily available. However, both guns and knives are everywhere in non-jailhouse Florida, so I wonder why someone would use a shiv in an attempted robbery.